X-ray diffractometry (XRD) is a well-known technique for studying the crystalline structure of matter. In XRD, a sample is irradiated by a monochromatic X-ray beam, and the locations and intensities of the diffraction peaks are measured. The characteristic diffraction angles and the intensity of the diffracted radiation depend on the lattice planes of the sample under study and the atoms that make up the crystalline material. For a given wavelength λ and lattice plane spacing d, diffraction peaks will be observed when the X-ray beam is incident on a lattice plane at angles θ that satisfy the Bragg condition: λ=2d sin θB. The angle θB that satisfies the Bragg condition is known as the Bragg angle. Distortions in the lattice planes due to stress, solid solution, defects or other effects lead to observable changes in the XRD spectrum.
XRD has been used, inter alia, for measuring characteristics of crystalline layers produced on semiconductor wafers. For example, U.S. Pat. No. 7,120,228, whose disclosure is incorporated herein by reference, describes a combined X-ray reflectometer and diffractometer. The described apparatus includes a radiation source, which is adapted to direct a converging beam of X-rays toward a surface of the sample. At least one detector array is arranged to sense the X-rays scattered from the sample as a function of elevation angle over a range of elevation angles simultaneously. In one configuration, the detector array senses the X-rays that are diffracted from the surface in a vicinity of a Bragg angle of the sample. A signal processor processes the output signals from the detector array so as to determine a characteristic of the surface layer of the sample.
Other systems for XRD measurement are described, for example, in U.S. Pat. Nos. 7,076,024 and 7,551,719, whose disclosures are also incorporated herein by reference.